Metallic products are superior in strength and cost compared with other materials, so are being used in a variety of fields such as offshore structures, ships, bridges, automobiles, industrial machinery, household electrical appliances, medical equipment, etc. Therefore, metallic products play important roles in industry.
However, the ultrahigh strength, fatigue resistance, wear resistance, and other characteristics required for metallic products are important characteristics not for the metallic products as a bulk, but in particular for the surface layers of the metallic products. In many cases, there is no need for the products as a bulk to have such characteristics.
Therefore, broad use is being made of the method of controlling the crystal structure of the surface layer of a metallic material so as to impart various superior properties to the material. Up to now, a succession of superior materials have been created with the introduction of each new process for the control of the crystal structure. In the future as well, there is a possibility of much more superior materials being created due to the introduction of new processes.
In recent years, it has become possible to refine the crystal structures of metallic materials to a nanometer (nm, 10−9 m) level size (for example refined to less than 100 nm), i.e., to achieve a nanocrystal structure, so as to obtain superior properties not achievable in the past, for example, ultrahigh strength.
As a method of obtaining a metallic material having a nanocrystal structure, there is known the method of once amorphize the metallic material and then converting it from a amorphous state to a crystalline state so as to obtain a nanocrystal structure.
As a method of amorphizing a metallic material, the method of high speed rapid cooling of the melt of the metallic material, sputter deposition, or other methods may be used.
If making the atomic configuration of a metallic material amorphous, unique properties not obtainable by a crystallined metal are obtained and a metallic material having high strength, corrosion resistance, high magnetic permeability, and other superior properties can be obtained.
By heat treating such an amorphous metallic material at a low temperature, it is possible to make fine nanometer (nm, 10−9 m) size crystals, that is, nanocrystals, precipitate. Further, it is possible to obtain a metallic material exhibiting properties more superior to an amorphous metal, for example, a metallic material exhibiting ultrahigh strength or a metallic material superior in magnetic characteristics (for example, see Japanese Unexamined Patent Publication (Kokai) No. 1-110707 or Japanese Patent No. 1944370).
The method of amorphizing a metallic material and then heat treating it at a low temperature to cause nanocrystals to precipitate in this way should be taken note of as a method for imparting superior properties and functions not achievable with conventional methods to a metallic material.
However, in providing metallic materials using this method for actual use, there have been the problems explained below.
First, as methods for obtaining metallic materials in the amorphous state, there are the method of high speed rapid cooling of the melt of the metallic material and the method of sputter deposition, but these methods involve high speed rapid cooling or deposition, so there are major restrictions on the shape or dimensions, and application to the production of shaped articles, structures, and metallic products of general shapes has been difficult.
Further, as the method of amorphizing a metallic material and causing nanocrystals to precipitate, in addition to the above-mentioned methods, the following method is known.
That is, it is possible to treat a powder of a metallic material by a ball mill etc., then work-harden the surface of the material to amorphize the material, then heat treat the material to obtain a metallic material with nanocrystals precipitated.
The thus produced metal powder may be used not only as an alloy powder of an amorphous metal as it is, but may also be press formed and used as shaped articles, structures, and metallic products of general shapes.
It becomes necessary to press form this powder at a high temperature to obtain a shaped article having sufficient strength for this purpose or weld such shaped articles to fabricate a desired structure.
However, if an alloy powder of an amorphous metal experiences a high temperature process, the powder will lose its nanocrystal structure and change to a large crystal structure. Therefore, it was not possible to obtain a shaped article, structure, or metallic product making use of the features of a nanocrystalline structure from a metal powder with nanocrystals precipitated.
Note that for example the specification of U.S. Pat. No. 6,171,415 discloses a method of modification of the fatigue strength by applying ultrasonic vibration to a welded joint zone, but does not disclose applying ultrasonic vibration to the surface layer of a metallic product to make it nanocrystalline.